Asserts and cors fix

@ngallagher87 for review in @bruce-alderson's absense.

Two very small changes:

1. Fixes a problem with `crossOrigin` function: if the request was not a CORS call (this should not cause an exception).
2. Sets up assertion options so that failed assertions are logged and treated as exceptions if `PRESTO_DEBUG` flag is set.

Also, I turned `PRESTO_DEBUG` on. How do you feel about that @ngallagher87? It should not be on in production but in development I think it should. I don't think we want to `gitignore` this file though.